1. Field
This disclosure relates generally to a wireless communication system and, more specifically, to techniques for reducing a cell identification falsing rate in a wireless communication system.
2. Related Art
In general, orthogonal frequency division multiplexing (OFDM) systems support high data rate wireless transmission using orthogonal channels. Typically, OFDM systems split data into N streams, which are independently modulated on parallel spaced subcarrier frequencies or tones. The frequency separation between subcarriers is 1/T, where T is the OFDM symbol time duration. Each symbol may include a guard interval (or cyclic prefix) to maintain the orthogonality of the symbols. OFDM systems have usually utilized an inverse discrete Fourier transform (IDFT) to generate a sampled (or discrete) composite time-domain signal for transmission.
Each base station (BS) in a wireless communication system has typically been designed to signal a unique cell identification (ID) in a transmitted downlink signal. Upon receipt of one or more downlink signals, a cell ID has been acquired by a subscriber station (SS) to uniquely identify a BS to which the SS has desired to communicate. For example, with reference to FIG. 1, a relevant portion of a downlink radio frame 100, provided by a third-generation partnership project-long term evolution (3GPP-LTE) compliant BS, is illustrated. As currently specified, a 3GPP-LTE compliant BS provides an associated cell ID in a hierarchical manner over a combination of downlink waveforms provided on a primary synchronization channel (P-SCH), at symbol 6 of slots 0 and 10 of each 10 millisecond radio frame, and a secondary synchronization channel (S-SCH), at symbol 5 of slots 0 and 10 of each 10 millisecond radio frame. As currently specified, the P-SCH and the S-SCH occupy seventy-two subcarriers (some of which are not utilized) centered around a DC subcarrier. Waveforms may be encoded on the P-SCH using generalized chirp like (GCL) sequences and on the S-SCH using binary sequences. In one proposed scheme, the S-SCH sequence has included a combination of two secondary short codes, i.e., a first secondary short code (SSC1) and a second secondary short code (SSC2). According to this scheme, three P-SCH sequences are employed to reduce the number of sequences an SS is required to search to acquire coarse timing and frequency alignment. An acquired P-SCH sequence has then been used by the SS for coherent detection of an S-SCH. The combination of the detected P-SCH and the detected S-SCH have then been decoded to provide a cell ID.
With reference to FIG. 2, a diagram 200 depicts a proposed cell ID assignment scheme for 3GPP-LTE that employs a secondary sequence group 212 that includes thirty-one SSC1s and thirty-one SSC2s to provide nine hundred sixty-one (31*31=961) total SSC1/SSC2 combinations. In this scheme, a subset 214 of one-hundred seventy of the SSC1/SSC2 combinations has been selected from the nine-hundred sixty-one total SSC1/SSC2 combinations and each of three P-SCH sequences 204, 206, and 208 (included in primary sequence group 202) has been associated with the subset 214 of one-hundred seventy SSC1/SSC2 combinations to provide a total of five-hundred ten (170*3=510) possible cell IDs. With reference to FIG. 3, a diagram 300 illustrates an SS 302 that may receive P-SCH sequences and SSC1/SSC2 combinations from multiple base stations (e.g., BS1, BS2, and BS3). As each of the P-SCH sequences is associated with the same subset of one-hundred seventy SSC1/SSC2 combinations, it is possible that the SS 302 (which may be a cell-edge SS) may detect the P-SCH from one BS (e.g., BS1) and the S-SCH from another BS (e.g., BS2), which may result in a false cell ID being detected by the SS 302. When an SS detects a false cell ID, it is unlikely that a communication link between the SS and a BS (associated with the false cell ID) will be successfully established.
In an attempt to randomize interference between neighboring cells and reduce the cell ID falsing rate (i.e., the possibility of an SS detecting an incorrect cell ID), one known scheme has proposed assigning a different scrambling code to each of three P-SCH sequences. In this case, an appropriate one of the scrambling codes (dependent upon the P-SCH sequence) is utilized to scramble the S-SCH sequence transmitted by a given BS. Similarly, another scheme has proposed scrambling each SSC2 sequence based on an associated SSC1 sequence to reduced the probability of an SS detecting an incorrect cell ID. While the use of scrambling codes normally reduces the possibility of an SS detecting a false cell ID, a cell ID falsing rate may still be less than desirable for many applications.